* stop using deprecated failure crate and its deps
* more uniformly detailed error messages
* an enum of the type of error, currently internal-only. I'm not
confident enough I understand what cases a caller might want to
distinguish, so I added a comment inviting input instead of
exposing it now.
* while I'm at it, separate connection context and message contexts.
This shrinks some of the data memmoved around in PacketItem and
CodecItem.
* add a couple tests
* fix#4: put the whole video frame (including multiple NALs) into
a single Bytes. This is simpler to use and speeds up the
depacketize/h264_aac_writevideo benchmark. It's also a behavior
change; now I include non-VUI data such as SPS/PPS/SEI into the
data. I think this is a better default but it might be worth
making customizable.
